The present invention relates to a method for producing a multilayer catalyst, to the multilayer catalyst produced by the method, and to the use of the catalyst for after-treatment of exhaust gases.
It has long been customary, especially with regard to motor vehicles, to subject the exhaust gas of a combustion motor to after-treatment using a catalyst. The task of the catalyst is to convert the pollutants generated during combustion, i.e., hydrocarbons (CmHn), carbon monoxide (CO), and nitrogen oxides (NOx), into the non-toxic substances carbon dioxide (CO2), water (H2O), and nitrogen (N2). The following oxidation and reduction reactions take place in this process:2CO+O2→2CO2 2C2H6+7O2→4CO2+6H2O2NO+2CO→N2+2CO2 
There are various types of catalysts. The best-known, aside from the three-way catalyst, are oxidation catalysts and NOx storage catalysts.
The three-way catalyst, also referred to as a controlled catalyst or “G-Kat,” has become standard equipment in a motor vehicle fitted with a combustion engine. In this context, the term “controlled” refers to the motor management of the combustion. The three-way catalyst can only be used in vehicles equipped with a combustion engine and lambda control. In a three-way catalyst, the oxidation of CO and HmCn and the reduction of NOx take place in parallel. This requires a constant air-fuel mixture at a stoichiometric ratio of lambda (λ) equal to 1.
In a combustion engine, the lambda probe ensures controlled combustion of the fuel. The lambda probe is used to determine the air-fuel ratio in the exhaust gas of the combustion engine. The measurement is based on the residual oxygen content present in the exhaust gas. The lambda probe is the main sensor in the control loop of the lambda control for catalytic after-treatment with a controlled catalyst and supplies the measured value to the motor control unit.
The lambda control establishes a desired lambda value in the exhaust gas of a combustion engine. In this context, lambda denotes the air-fuel ratio, which is the ratio of the mass of air available for combustion to the minimal stoichiometric mass of air required for complete combustion of the fuel. At the stoichiometric fuel ratio, exactly the amount of air required for complete combustion of the fuel is present. This is called λ=1. If more fuel is present, the mixture is called rich (λ<1), whereas an excess of air being present corresponds to a lean mixture (λ>1). If there is any deviation from the stoichiometric air-fuel ratio towards an excess of air, i.e., lean region, not all nitrogen oxides are decomposed, since the requisite reducing agents are being oxidized earlier. In the rich region, i.e., air deficit, not all hydrocarbons and not all of the carbon monoxide are decomposed.
The air-fuel equivalence ratio lambda, also called “air excess,” air excess number,” or “air ratio” for short, is a parameter of combustion technology. This parameter provides some feedback concerning the progress of the combustion, temperatures, generation of pollutants, and the efficiency. Proper fine-tuning of carburetor or fuel injection facility, and thus the adjustment of lambda, has a major impact on motor performance, fuel consumption, and the emission of pollutants.
Combustion engines are usually controlled to a narrow range of approx. 0.97<λ<1.03. The range within these thresholds is called the lambda window. The best reduction of all three types of pollutants is attained within this window. At high motor performance, operating the engine with a rich mixture, and therefore colder exhaust gas, prevents the exhaust components, such as manifold, turbo-charger, and catalyst, from overheating.
To attain a value of λ=1 in operation, sufficient oxygen must be available in the catalyst in order to carry out the oxidation-reduction reactions indicated above. On the other hand, oxygen released during the reduction must be bound for the reduction of the nitrogen oxides to nitrogen to take place. Three-way catalysts usually contain an oxygen reservoir that is charged with oxygen at oxidizing conditions and can release oxygen again at reducing conditions.
In addition to the oxygen reservoir, a catalyst often also comprises at least one noble metal; usually this will be platinum, palladium and/or rhodium. If aluminum oxide is also used in a catalyst, it is important to ensure that the rhodium does not become applied onto the aluminum oxide. At elevated temperatures, the rhodium adsorbs to the porous structure of the aluminum oxide and is therefore no longer available for the actual catalytic reaction. Accordingly, EP 1053779 A1 describes a catalyst in which the catalytically active layer comprises a cerium complex oxide and a zirconium complex oxide. While palladium is situated on the cerium complex oxide, platinum and rhodium are applied onto the zirconium complex oxide.
DE 10024 994 A1 describes a catalyst in which the noble metals are applied onto a substrate as separate layers. The catalyst comprises a first coating layer formed on a heat-resistant substrate and a second coating layer formed on the first coating layer. The first coating layer contains aluminum oxide bearing palladium; the second coating layer contains cerium zirconium complex oxides bearing both platinum and rhodium.
During the production of the catalysts, it is customary to apply a first layer containing palladium onto the substrate. This layer is then annealed at a temperature of approx. 550° C. Subsequently, a second layer containing platinum and/or rhodium is applied and annealed at a temperature of approx. 450° C. A pertinent method is described, for example, in US 2009/257933 A1, US 2008/044330 A1 or WO 02/083301 A2. A temperature of 600° C. for annealing of the first and second layer is described in US 2005/0255993 A1.
Annealing the layers reduces the noble metal that is obtained in the form of a salt. As a result, approximately spherical particles of the corresponding noble metal are obtained. The noble metal particles are the active centers of the catalyst and the actual oxidation and/or reduction reactions take place on them. Accordingly, the production method has an impact on the catalytic activity of the catalyst. Obviously, the catalytic activity should be as high as possible so as to minimize the emission of pollutants, such as nitrogen oxides NOx, carbon monoxide CO, and hydrocarbons HC, in the exhaust gas.